1. Field of the Invention
The present invention relates to an ink, in particular, an ink suitable for use in ink-jet recording, a recording method making use of such an ink and instruments including ink-jet recording apparatus making use of such an ink.
2. Related Background Art
An ink-jet recording system is a method in which recording is conducted by flying out droplets of an ink to apply the droplets to a recording material such as paper. In particular, according to the methods disclosed in Japanese Patent Publication Nos. 61-59911, 61-59912 and 61-59914 by the present applicant, in which an electrothermal converter is used as an ejection-energy supply means to apply thermal energy to an ink so as to generate bubbles, thereby ejecting out droplets of the ink, formation of a high-density multi-orifice in a recording head can be realized with ease, and high-resolution and high-quality images can be recorded at high speed.
Of image-forming processes according to such an ink-jet recording system, a technique which has especially acquired an importance in recent years is an image-forming process which permits highly fine, full-color recording, which is good in coloring and free of any leathering or bleeding, even on non-coated paper such as paper for electrophotography, paper for reporting, notepaper, letter paper, bond paper and continuous business forms (these will hereinafter be called "plain paper" collectively), which are commonly used in offices and homes. The design of ink is one of important points for achieving such a technique.
However, the conventional inks used in ink-jet recording generally comprise, as a principal component, water, and include additionally a water-soluble high boiling solvent such as glycol for the purpose of preventing drying and clogging, and the like. When such an ink has been used to conduct recording on plain paper, there have been disadvantages that the ink penetrates into the interior of the recording paper, resulting in failure to obtain images having sufficient color depth, and moreover, unevenness of color strength occurs on images, which appears to be attributed to the uneven distribution of the filler and size on the surface of the recording paper. In particular, when a color image is intended to form, plural inks of different colors are overlapped one after another before the time required to fix the inks goes on. Therefore, color bleeding and uneven color mixing have occurred at portions of boundaries between different colors in the resulting image (hereinafter referred to as "bleeding" simply), resulting in failure to obtain a satisfactory image.
Japanese Patent Application Laid-Open No. 55-65269 discloses the use of inks added with a compound capable of enhancing their penetrability, such as a surfactant, and Japanese Patent Application Laid-Open No. 55-66976 discloses the use of inks comprising, as a principal component, a volatile solvent. It is also disclosed in Japanese Patent Application Laid-Open No. 55-57862 to add a strongly basic substance to an ink so as to chemically dissolve a size and pulp contained in a paper sheet, thereby controlling the spreading of dots and the easiness of absorption. Further, Japanese Patent Application Laid-Open No. 58-13675 discloses the addition of polyvinyl pyrrolidone having a molecular weight not lower than 40,000 to an ink, or Japanese Patent Application Laid-Open No. 3-172362 discloses a method of controlling the spreading of dots and the easiness of absorption by adding a specific microemulsion to an ink.
However, the above-mentioned prior art techniques have involved such problems as described below.
The method of enhancing the penetrability of an ink into the interior of paper by the addition of a surfactant can improve the fixing ability of the ink, but involves a problem that since a recording agent also penetrates deeply, the coloring ability of the ink is lowered. In addition, since the wettability to the surface of the recording paper is also improved, the ink tends to spread, resulting in reduction of resolution and occurrence of featbering. Such a method has not hence been preferred. Further, if a volatile solvent is used in an ink, the evaporation rate of the solvent on recording paper is actually lower than the rate of the occurrence of bleeding. Therefore, the bleeding cannot be completely prevented under circumstances. In addition, such an ink is liable to cause clogging due to the evaporation of the solvent in a nozzle of a recording head. The method has not also been preferred from the viewpoint of long-term reliability on these points.
The method in which a strongly basic substance is added to an ink is insufficient to solve problems of both featbering and fixing ability against paper making use of a certain size, for example, neutralized paper, and also has a problem of safety for human body. Further, the mere addition of a polymer or a certain emulsion to an ink fails to exhibit sufficient effects in such a range that the recording density of the ink becomes greater upon full-color recording or the like.
As a problem involved in the prior art, may be mentioned the following suitability of ink for recording on plain paper in addition to the above-described problems. Namely, there are a wide variety of kinds in paper sheets collectively called the plain paper. It may safely be said that their factors such as ink adsorptiveness, surface smoothness, the kind, thickness and length of pulp fibers, and the distribution of various additives such as a surface size and an internal size on the surface thereof, which are considered to participate in suitability for ink-jet recording, all differ between different kinds of paper. As a result, there has been a problem that quality of character such as degree of featbering, evenness of solid printed areas, ink fixing ability, degree of bleeding on color images, and the like greatly vary according to the kind of paper.